The present invention relates in general to the field of heat transfer, and, in particular, to a heat transfer assembly and method that efficiently removes heat from electronic components.
Electronic components, such as microprocessors and integrated circuits, must operate within certain specified temperature ranges to perform efficiently. Excessive heat degrades electronic component performance, reliability, life expectancy, and can even cause failure. Heat sinks are widely used for controlling excessive heat. Typically, heat sinks are formed with fins, pins, or other similar structures to increase the surface area of the heat sink and thereby enhance heat dissipation as air passes over the heat sink. In addition, it is not uncommon for heat sinks to contain high performance structures, such as vapor chambers and/or heat pipes, to enhance further heat transfer. Heat sinks are typically formed of metals, such as copper or aluminum. More recently, graphite-based materials have been used for heat sinks because such materials offer several advantages, such as improved thermal conductivity and reduced weight.
Electronic components are generally packaged using electronic packages (i.e., modules) that include a module substrate to which the electronic component is electronically connected. In some cases, the module includes a cap (i.e., a capped module) which seals the electronic component within the module. In other cases, the module does not include a cap (i.e., a bare-die module) wherein the electronic component directly engages the heat sink.
Bare-die modules are generally preferred over capped modules from a thermal performance perspective. In the case of a capped module, a heat sink is typically attached having a thermal interface gap material existing between a bottom surface of the heat sink and a top surface of the cap, and another thermal interface gap material existing between a bottom surface of the cap and a top surface of the electronic component. In the case of a bare-die module, a heat sink has a thermal interface gap material existing between a bottom surface of the heat sink and a top surface of the electronic component. Bare-die modules typically exhibit better thermal performance than capped modules because bare-die modules eliminate two sources of thermal resistance present in capped modules, i.e., the thermal resistance of the cap and the thermal resistance of the thermal interface gap material between the cap and the electronic component. Accordingly, bare-die modules are typically used to package electronic components, such as semiconductor chips, that require high total power dissipation.
Heat sinks are attached to modules using a variety of attachment mechanisms, such as clamps, bolts, and other hardware. The attachment mechanism typically applies a force that maintains a thermal interface gap, i.e., the thickness of the thermal interface gap material extending between the heat sink and the module. In the case of a capped module, the cap protects the electronic component from physical damage from the applied force. In the case of a bare-die module, however, the applied force is transferred directly through the electronic component itself onto the bare-die module. Consequently, when bare-die modules are used, the attachment mechanism typically applies a compliant force to decrease stresses on the electronic component.
FIG. 1 illustrates an example of a prior art attachment mechanism for attaching a heat sink to a bare-die module. There is illustrated a circuit board assembly 100 that includes a printed circuit board 105, and a bare-die module 110. Bare-die module 110 includes a module substrate 115, an electronic component, such as a semiconductor chip 120, and an electronic connection 125. Semiconductor chip 120 is electrically connected to module substrate 115. Electronic connection 125, which electrically connects printed circuit board 105 to module substrate 115, may be a pin grid array (PGA), a ceramic column grid array (CCGA), a land grid array (LGA), or the like. Semiconductor chip 120 is thermally connected to a heat sink 130 through a thermal interface gap material 135. The thermal interface gap material maybe a layer of a thermally conductive medium, such as thermal paste, grease, oil, or other high thermal conductivity material. Typically, the thermal interface gap material 135 is relatively thin so that it may effectively transfer heat away from the bare-die module 110 and toward heat sink 130. The thickness of thermal interface gap material 135 extending between heat sink 130 and semiconductor chip 120 is referred to as the thermal interface gap.
Heat sink 130 is attached to bare-die module 110 using bolts 140. Bolts 140 pass through thru-holes 131 in heat sink 130 and thru-holes 106 in printed circuit board 105 and are threaded into threaded-holes 146 in a backside bolster 145. Typically, bolts 140 are arranged one at each corner of the electronic component 120, or one on each side of the electronic component 120. Bolts 140 are tightened by threading a threaded portion of bolts 140 into threaded-holes 146 in backside bolster 145. As bolts 140 are tightened, heat sink 130 engages semiconductor chip 120 through thermal interface gap material 135. Additional tightening of bolts 140 causes deflection (bowing) of the printed circuit board 105, which applies a compliant force to bare-die module 110. More particularly, printed circuit board 105 is slightly flexed in a concave-arc fashion with respect to bare-die module 110.
Presently, some computer systems use multiple chip assemblies that require high and stable loading. The multiple chip assemblies have very thin thermal gaps filled with a thermal interface layer to establish thermal engagement with the heat sink. However, due to physical chip height variations, the noted deflections can lead to non-planar thermal interfaces being formed, thereby resulting in possible thermal degradation of the CPU. Moreover, the detrimental effects due to differences in chip height get more pronounced with power cycling loading. During power cycling loading, the chip heat dissipation results in a temperature gradient across the chip, thermal interface layer, and heat sink base. This temperature gradient has thermal transients around the vicinity of the chip, thereby resulting in distortion or relative movement of the heat sink base to the chip. These effects are further compounded when a bare-die solution is used. Consequently, thermal efficiency is compromised. Accordingly, continuing efforts are being made to improve thermal efficiency in such situations.